Former Los Angeles and New York police chief William Bratton has been busy the last few days preparing for a new role overseas. The mayor of Philadelphia, Michael Nutter, has been busy as well, walking the streets of his city—recently the scene of flash mobs committing ugly assaults along with robbery—and making clear his refusal to tolerate any of it. Mr. Bratton has been tapped, in the wake of the London riots, to serve as British Prime Minister David Cameron’s adviser on crime control, policing and related matters. He’d do well to take the Philadelphia mayor with him, at least in spirit.

The eruptions of flash mobs in a few American cities will seem no great matter compared to the four-day reign of terror loosed over London, Manchester, Birmingham and other British cities last week. Still there’s no missing their common factors—the same gleeful assaultiveness, the party atmosphere, the unembarrassed rationalizations for criminal brutality. In both cases, the rationalizations for criminality took a much further leap into the absurd than is usual for such efforts—no small distinction.

Former London Mayor Ken Livingstone explained that government spending cuts had caused the trouble, especially in Tottenham, where the problems began. But Labour Member of Parliament John McDonnell easily topped that old social-causes standby with his announcement, reported in the London Telegraph, that Britain was reaping what had been sown: the alienated young had been copying “the ethos of looting bankers.”

The looting-bankers defense turns out to have been popular across the ocean as well. Washington Post columnist Courtland Milloy had also divined—in a column last Tuesday titled “From London to Philadelphia, youths erupting over the theft of their futures”—that the bankers and their kind were to blame for the rioters in Britain and the flash mobs in America.

Think of their actions in both places, he advised, as the “alley version of the Wall Street bum rush and rip-off.” First, explained Mr. Milloy, “there were flash mobs of bankers and mortgage lenders picking pockets, looting businesses, taking over homes.” Now the young were using “a crude version of the same tactics” as they struck out in anger.

Mr. Milloy was apparently offended, too, that the black mayor of Philadelphia, Mr. Nutter, had advised young black men, after the flash riot there, that they would do well to find jobs. But, Mr. Nutter told them, they couldn’t come to interviews with hair uncombed, a pick in the back, shoes untied, full arm and neck tattoos, and their pants half down. They weren’t going to be hired looking like that, the mayor noted, “because you look like you’re crazy.”

Mr. Milloy, who described the mayor’s remarks as even “nuttier” than others he had heard, had evidently found the remarks insensitive to these young. Youngsters who rioted and took part in flash mobs felt disrespected—that was why, Mr. Milloy explained, they were “threatening to destroy what they can’t have. And why not?” It was a strategy that had worked, he added, for tea party anarchists.

For some reason, Mr. Milloy neglected to mention a key element in the violence of these youths—namely that it has been entirely and indisputably racial in its targets. The attackers are invariably young blacks, the people they assault invariably whites. This was true in Philadelphia as it was in Wisconsin, in Chicago and everywhere else that has seen these mobs.

It’s not difficult to imagine the column Mr. Milloy would have written if the story were reversed—with gangs of young white males beating and kicking black men and women in the streets of America. It’s a good bet that column would have concerned itself with matters other than the anger of these young, robbed of their futures by bankers and Wall Street.

It’s a good bet too that Mr. Nutter’s entirely different response to the rioters in his city—a powerful display of moral authority—will be remembered, to his great credit. In tones of steel he described the flash mob members who had set upon innocent people and beaten them, as cowards. On Friday, after Philadelphia police arrested 50 of the rioters, the mayor announced the imposition of strict curfews for all young people. “Get a watch,” he instructed parents. “We’re as serious as a heart attack about this,” he informed them. “The curfew is the curfew.” No one listening could have imagined otherwise.

Back in England, meanwhile, they were still cleaning up. The families of the dead still mourned, and the grousing by police opposed to the appointment of the American, Mr. Bratton, was becoming increasingly bitter. Former Scotland Yard official John O’Connor told the BBC that what Americans did to control crime is “they locked people up. . . . We haven’t got the heart for that over here.” A former police union head argued, “Americans police by force. We don’t want to do that here.”

Perhaps. But there were indications—after the outrages that had shaken their lives and stunned most of the world—that more than a few British citizens would have found it in their hearts to see criminals locked up. As many of them as possible, and for a long time.

It seemed best for a while last week to avoid certain historical images—those well-known pictures of Londoners marching briskly through bombed-out streets to their day jobs after another night of Luftwaffe attacks, of the shops with their shattered windows and rubble for storefronts, always open for business. Haunting images today for their reminder of the nation that was, and the startlingly different one it has become today.

It was a transformation made strikingly clear in the four-day rule-by-mob. It showed in the confident thuggery of the looters, and even more so in the fact that a number of regular job-holding Britons took part in the action. Others opined in emails that it was all a matter of social justice—after all, big department stores that had been ransacked had plenty of insurance. Nowhere was the transformation clearer than in the helplessness of citizens largely left to the mercy of marauders in the first stages of the riots. There would be little help initially from the police—outmanned, taken by surprise, uncertain of what they were allowed to do and not do in this situation. What violation of human rights regulations might they be charged with?

In the spring of 1941, Noel Coward sat in a bomb-damaged railroad station composing that exquisitely buoyant song, “London Pride,” a tribute to his beloved city and of course to England itself. Citizen of a nation at war, and in imminent peril, Coward dropped into the gaiety of that tribute a different mood.

In our city darkened now

Street and square and crescent

We can feel our living past in our shadowed present.

Those words came to mind while the show of horrors across the ocean played out: the arsonists, the unstoppable army of destroyers, the three young men killed because they had assembled to protect a local business, the old man beaten—he would die—for his attempt to stomp out the fire set by marauders.

Still there remained, in the city now darkened, reminders of that other England. It was there in the brigades of people who turned out to wield brooms, sweep up the rubble and glass, clear ruins. It was there too in those who had made it clear that they expected punishment for crimes committed—so many people signed a government petition on this subject that the site crashed—and in those who had shown they weren’t prepared to tolerate any recurrence of this trip to the abyss, or any excuses for it. The prime minister and his men would be well-advised to listen.

Ms. Rabinowitz is a member of the Journal’s editorial board.